Differential condenser



May 1935- B. VAN DER POL ET AL 2,000,678

DIFFERENTIAL CONDENSER Filed June 4, 1932 INVENTOR BALTHASAR VAN DER POL Y ROM VELDHUYZEN v 7 WM ATTORNEY Patented May 7, 1935 PATENT OFFICE DIFFERENTIAL CONDENSER Balthasar van der Pol and Romyn Veldhuyzen, Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application June 4, 1932, Serial No. 615,292 In Germany June 12, 1931 5 Claims.

The invention relates to a differential condenser and to a circuit-arrangement therefor which serves more particularly for controlling the sound volume with wireless apparatus. The amplitude of the oscillations acting on the grid of one of the valves can be controlled by means 01 such a condenser. This control afiords various advantages over that in which the negative bias of one or more grids of the high-frequency valve is varied. It affords inter alia the advantage oi avoiding the so-called cross-modulation which is due to the fact that the valves operate on the curved part of the characteristic curve. It has previously been suggested to utilize differential condensers for the volume control. The known condensers have, however, the inconvenience that they act on the tuning of the apparatus and that it is difllcult to reduce the sound volume down to zero. In many cases, for example, if the receiver is arranged in the neighborhood of a transmitter. a far-reaching reduction or the amplitude of the incoming oscillations is required.

The invention has for its object to remove the said inconveniences.

A diiIerential condenser generally comprises three plates of which two are mounted so as to be stationary whereas the third one is movable. According to the invention, these plates are arranged in such manner that the fixed plates are connected to that point of the input circuit which is not earthed and to the aerial respectively whereas the third plate which is arranged between the two fixed plates is connected to earth. In this manner one may obtain the adjustment of the sound volume to zero.

The movable plate may comprise an insulating part whose surface covered by the fixed plates increases when sliding-out the conductive plate, and vice versa. By giving the insulating part a determined thickness and a determined dielectric constant, it is obtained that the capacity between one of the fixed plates and the movable plate decreases as rapidly as increases the capacity between the two fixed plates, and conversely. The total value always remains constant in this case.

The invention is explained hereinafter by way of example with reference to the accompanying drawing in which,

Fig. 1 represents a differential condenser of known type connected in the aerial circuit of a wireless receiving apparatus;

Fig. 2 represents such a circuit-arrangement in which use is made of a differential condenser according to the invention;

Fig. 3 is a diagrammatic section of a condenser according to the invention; and,

Fig. 4 shows the construction of the separate plates of the condenser shown in Fig. 3.

In the arrangement shown in Fig. 1 a differential condenser of known type is connected in the aerial circuit of a wireless receiving apparatus. This condenseressentially consists of three plates I, 2 and 3 of which the plate I is movable and is connected to that point of the input circuit which is not earthed. The plates 2 and 3 are connected to the earth and to the aerial respectively. If the plate I is adjusted in such manner that the capacity between the plates I and 3 has its maximum value, the capacity between the plates I and 2 has its minimum value and the oscillations coming from the aerial and acting on the grid of the first valve are consequently strongest. The sound volume is adjusted in this case to the maximum value. In the opposite case the sound volume has its minimum value. In all cases it is, however, not possible to bring the sound volume down to zero since between the plates l and 3 there remains always a slight capacity.

According to Fig. 2, the aerial circuit has connected in it a differential condenser according to the invention whereby the above-mentioned drawback is removed. In this case the movable plate 2 is earthed, for example, by connecting it directly to the earth terminal of the apparatus. The connection is preieraby made as short as possible. The fixed plates I and 3 are connected to the point of the aerial circuit which is not earthed and to the aerial respectively. It is evident that the condenser may also be connected in any of the high-frequency intermediate circuits. The plate connected to earth is movable between the fixed plates and preferably has a slightly larger surface than these so that in the zero position the aerial is perfectly screened from the input circuit of the apparatus. The volume control may, however, not exert any influence on the tuned aerial circuit. The aerial capacity generally is rather high and there exists the condition that the total value of the capacity between the plates I and 2 and the capacity between the plates I and 3 should always remain constant. For this purpose the movable plate 2 is provided with an insulating part 4. When sliding-out the plate 2, the capacity of the plate I relatively to the plate 2 decreases whereas the capacity of the plate I relatively to the plate 3 increases. Since, however, the distance between the plates I and 3 is larger than that between the plates l and 2, the increase of the capacity between the plates l and 3 would be smaller than the decrease of the capacity between the plates and 2 if there were no dielectric. The

part 4, which is united with the plate 2 and which acts as the dielectric, decreases, as it were, the distance between the plates I and 3. By a suitable choice of the dielectric constant and of the thickness of the part 4 it may be obtained that the increase of the capacity between the plates 5 and 2 is as large as the decrease of the capacity between the plates and 2.

In the form of execution of the condenser as shown in Figures 3 and 4 the plate 3, which is connected to the aerial, has arranged around it a screen 5 connected, as the plate 2, to earth owing to which the production of a leakage field is counteracted. The plates have a semi-circular shape and the plate 2 is preferably made larger than the plates 2 and 3. The plate 2 together with the part which acts as a dielectric, are adapted to turn about a spindle. 6 and I are plates of insulating material to which the fixed plates are secured.

We claim:-

1. In a 0 receiving system including an antenna circi associated with the input of radio apparatus through a coupling condenser, said condenser c "isinthree cooperating plates, two thereof b J fixed and the third being adiustably ass ciated with respect to the other two thereof, said cooperating plates acting to control the degree of coupling between the antenna circuit and the radio apparatus input, said movable plate comprising an insulating part arranged with respect to the fixed plates so that the surface thereof increases when the variable plate is adjusted in one direction and vice versa, said insulating part being shaped and possessing such dielectric characteristics that the total value of the capacity between one or" the fixed plates and the movable plate and the mutual capacity between the fixed plates remains constant.

2. In the system as described in claim 1 Wherein the fixed plates, the movable plate and the insulating part are semi-circular in shape and coaxially arranged relatively to one another, said movable plate together with the insulating part being adapted to turn above a spindle passing through the center thereof.

3. A system as claimed in claim 1 wherein a conductive screen is provided around one of the fixed plates, said screen being connected to the movable plate.

4. A variable coupling condenser arrangement for coupling a source of energy to a tuned circuit and arranged so that the tuning of the tuned circuit is not affected by variations in the coupling condenser comprising, three cooperating elements two of said elements being fixed and the third being interposed between and adjustably associated with respect to the other two thereof, said cooperating elements acting to determine the degree of coupling between the source and the tuned circuit said adjustable member comprising a plate made up so that a portion thereof is constructed of dielectric material while another portion is constructed of shielding material, the dielectric material being proportioned with respect to the shielding material so that the total value of the capacity between one of the fixed plates and the movable plate and the mutual capacity between the fixed plates remains constant despite variations in the coupling value of the condenser.

5. A condenser comprising a pair of spaced fixed plates and a movable plate interposed between the two fixed plates the movable plate comprising a shielding portion and a dielectric portion, the dielectric portion being shaped with respect to the shielding portion so that the total value of the capacity between one of the fixed plates and the movable plate and the mutual capacity between the fixed plates remains constant irrespective of adjustments of the movable plate with respect to the fixed plates.

BALTHASAR VAN DER POL. ROMYN VELDHUYZEN. 

